In recent years, stereoscopic video display devices (so-called autostereoscopic TVs), through which a viewer can view video auto-stereoscopically without using special glasses, are becoming widely used. This kind of stereoscopic video display device displays a plurality of images whose viewpoints are different from each other. Output directions of light beams from the images are controlled by, for example, a parallax barrier, a lenticular lens, or the like, so that the light beams are guided to the eyes of the viewer. If the position of the viewer is appropriate, the viewer views different parallax images with the left eye and the right eye, and thus, the viewer can recognize video stereoscopically.
However, there is a problem that video displayed on the autostereoscopic TV is not viewed stereoscopically depending on the position of the viewer. When the viewer views normal video, it may be difficult to determine whether or not the viewer is located at an appropriate position from which the viewer can view the video stereoscopically.